An E. coli protein required for integration of bacteriophage lambda DNA into the host chromosome has been purified and characterized as to subunit and genetic makeup. Footprinting experiments reveal that this protein binds specifically to the viral recombining site and may stabilize the interaction of a viral protein, Int. that also must interact with this site. A variant Int protein that may have an altered affinity for or dependence on the host protein has been purified; characterization of its interaction with the components of the recombination system is underway. Indirect evidence, based on the formation of knotted DNA during recombination, indicates that recombination proteins and the DNA of one of the recombining sites may be organized into a structure like that of the nucleosome subunits of chromatin.